I've never been seated early at Ohana, regardless of the time of day or the time of year. Not saying its impossible, but I wouldn't count on it.
Also just want to point out, incase you have children that are looking forward to a fun dinner, that often times they stop the fun activities around 9:30ish. We've dined at Ohana's around 9:30 twice and on both of those occasions the fun activities were done for the day. When we asked the CMs on both of those trips we were told that they typically quit doing that around 9:00/9:30. Again, maybe you'll be lucky and this is not the case, but just want you to be prepared just incase.